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Mon, 14 Dec 2020 09:28:55 -0800 (PST) To: David Laight , Al Viro Cc: Christoph Hellwig , Jens Axboe , "linux-block@vger.kernel.org" , "linux-kernel@vger.kernel.org" References: <9bc27cb3ef6ab49b6b2ccee3db6613838aee17af.1605799583.git.asml.silence@gmail.com> <20201119170340.GA6179@infradead.org> <20201211020100.GB107834@ZenIV.linux.org.uk> <857a3161-fbd5-5ff8-d733-ca57923302b5@gmail.com> From: Pavel Begunkov Autocrypt: addr=asml.silence@gmail.com; prefer-encrypt=mutual; keydata= mQINBFmKBOQBEAC76ZFxLAKpDw0bKQ8CEiYJRGn8MHTUhURL02/7n1t0HkKQx2K1fCXClbps bdwSHrhOWdW61pmfMbDYbTj6ZvGRvhoLWfGkzujB2wjNcbNTXIoOzJEGISHaPf6E2IQx1ik9 6uqVkK1OMb7qRvKH0i7HYP4WJzYbEWVyLiAxUj611mC9tgd73oqZ2pLYzGTqF2j6a/obaqha +hXuWTvpDQXqcOZJXIW43atprH03G1tQs7VwR21Q1eq6Yvy2ESLdc38EqCszBfQRMmKy+cfp W3U9Mb1w0L680pXrONcnlDBCN7/sghGeMHjGKfNANjPc+0hzz3rApPxpoE7HC1uRiwC4et83 CKnncH1l7zgeBT9Oa3qEiBlaa1ZCBqrA4dY+z5fWJYjMpwI1SNp37RtF8fKXbKQg+JuUjAa9 Y6oXeyEvDHMyJYMcinl6xCqCBAXPHnHmawkMMgjr3BBRzODmMr+CPVvnYe7BFYfoajzqzq+h EyXSl3aBf0IDPTqSUrhbmjj5OEOYgRW5p+mdYtY1cXeK8copmd+fd/eTkghok5li58AojCba jRjp7zVOLOjDlpxxiKhuFmpV4yWNh5JJaTbwCRSd04sCcDNlJj+TehTr+o1QiORzc2t+N5iJ NbILft19Izdn8U39T5oWiynqa1qCLgbuFtnYx1HlUq/HvAm+kwARAQABtDFQYXZlbCBCZWd1 bmtvdiAoc2lsZW5jZSkgPGFzbWwuc2lsZW5jZUBnbWFpbC5jb20+iQJOBBMBCAA4FiEE+6Ju PTjTbx479o3OWt5b1Glr+6UFAlmKBOQCGwMFCwkIBwIGFQgJCgsCBBYCAwECHgECF4AACgkQ Wt5b1Glr+6WxZA//QueaKHzgdnOikJ7NA/Vq8FmhRlwgtP0+E+w93kL+ZGLzS/cUCIjn2f4Q Mcutj2Neg0CcYPX3b2nJiKr5Vn0rjJ/suiaOa1h1KzyNTOmxnsqE5fmxOf6C6x+NKE18I5Jy xzLQoktbdDVA7JfB1itt6iWSNoOTVcvFyvfe5ggy6FSCcP+m1RlR58XxVLH+qlAvxxOeEr/e aQfUzrs7gqdSd9zQGEZo0jtuBiB7k98t9y0oC9Jz0PJdvaj1NZUgtXG9pEtww3LdeXP/TkFl HBSxVflzeoFaj4UAuy8+uve7ya/ECNCc8kk0VYaEjoVrzJcYdKP583iRhOLlZA6HEmn/+Gh9 4orG67HNiJlbFiW3whxGizWsrtFNLsSP1YrEReYk9j1SoUHHzsu+ZtNfKuHIhK0sU07G1OPN 2rDLlzUWR9Jc22INAkhVHOogOcc5ajMGhgWcBJMLCoi219HlX69LIDu3Y34uIg9QPZIC2jwr 24W0kxmK6avJr7+n4o8m6sOJvhlumSp5TSNhRiKvAHB1I2JB8Q1yZCIPzx+w1ALxuoWiCdwV M/azguU42R17IuBzK0S3hPjXpEi2sK/k4pEPnHVUv9Cu09HCNnd6BRfFGjo8M9kZvw360gC1 reeMdqGjwQ68o9x0R7NBRrtUOh48TDLXCANAg97wjPoy37dQE7e5Ag0EWYoE5AEQAMWS+aBV IJtCjwtfCOV98NamFpDEjBMrCAfLm7wZlmXy5I6o7nzzCxEw06P2rhzp1hIqkaab1kHySU7g dkpjmQ7Jjlrf6KdMP87mC/Hx4+zgVCkTQCKkIxNE76Ff3O9uTvkWCspSh9J0qPYyCaVta2D1 Sq5HZ8WFcap71iVO1f2/FEHKJNz/YTSOS/W7dxJdXl2eoj3gYX2UZNfoaVv8OXKaWslZlgqN jSg9wsTv1K73AnQKt4fFhscN9YFxhtgD/SQuOldE5Ws4UlJoaFX/yCoJL3ky2kC0WFngzwRF Yo6u/KON/o28yyP+alYRMBrN0Dm60FuVSIFafSqXoJTIjSZ6olbEoT0u17Rag8BxnxryMrgR dkccq272MaSS0eOC9K2rtvxzddohRFPcy/8bkX+t2iukTDz75KSTKO+chce62Xxdg62dpkZX xK+HeDCZ7gRNZvAbDETr6XI63hPKi891GeZqvqQVYR8e+V2725w+H1iv3THiB1tx4L2bXZDI DtMKQ5D2RvCHNdPNcZeldEoJwKoA60yg6tuUquvsLvfCwtrmVI2rL2djYxRfGNmFMrUDN1Xq F3xozA91q3iZd9OYi9G+M/OA01husBdcIzj1hu0aL+MGg4Gqk6XwjoSxVd4YT41kTU7Kk+/I 5/Nf+i88ULt6HanBYcY/+Daeo/XFABEBAAGJAjYEGAEIACAWIQT7om49ONNvHjv2jc5a3lvU aWv7pQUCWYoE5AIbDAAKCRBa3lvUaWv7pfmcEACKTRQ28b1y5ztKuLdLr79+T+LwZKHjX++P 4wKjEOECCcB6KCv3hP+J2GCXDOPZvdg/ZYZafqP68Yy8AZqkfa4qPYHmIdpODtRzZSL48kM8 LRzV8Rl7J3ItvzdBRxf4T/Zseu5U6ELiQdCUkPGsJcPIJkgPjO2ROG/ZtYa9DvnShNWPlp+R uPwPccEQPWO/NP4fJl2zwC6byjljZhW5kxYswGMLBwb5cDUZAisIukyAa8Xshdan6C2RZcNs rB3L7vsg/R8UCehxOH0C+NypG2GqjVejNZsc7bgV49EOVltS+GmGyY+moIzxsuLmT93rqyII 5rSbbcTLe6KBYcs24XEoo49Zm9oDA3jYvNpeYD8rDcnNbuZh9kTgBwFN41JHOPv0W2FEEWqe JsCwQdcOQ56rtezdCJUYmRAt3BsfjN3Jn3N6rpodi4Dkdli8HylM5iq4ooeb5VkQ7UZxbCWt UVMKkOCdFhutRmYp0mbv2e87IK4erwNHQRkHUkzbsuym8RVpAZbLzLPIYK/J3RTErL6Z99N2 m3J6pjwSJY/zNwuFPs9zGEnRO4g0BUbwGdbuvDzaq6/3OJLKohr5eLXNU3JkT+3HezydWm3W OPhauth7W0db74Qd49HXK0xe/aPrK+Cp+kU1HRactyNtF8jZQbhMCC8vMGukZtWaAwpjWiiH bA== Subject: Re: [PATCH 2/2] iov_iter: optimise iter type checking Message-ID: <5b65de70-19db-4572-d122-df65191ab098@gmail.com> Date: Mon, 14 Dec 2020 17:25:37 +0000 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.3.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 14/12/2020 10:28, David Laight wrote: > From: Pavel Begunkov >> Sent: 13 December 2020 22:33 >> >> On 11/12/2020 02:01, Al Viro wrote: >>> On Thu, Nov 19, 2020 at 05:12:44PM +0000, Pavel Begunkov wrote: >>>> On 19/11/2020 17:03, Christoph Hellwig wrote: >>>>> On Thu, Nov 19, 2020 at 03:29:43PM +0000, Pavel Begunkov wrote: >>>>>> The problem here is that iov_iter_is_*() helpers check types for >>>>>> equality, but all iterate_* helpers do bitwise ands. This confuses >>>>>> a compiler, so even if some cases were handled separately with >>>>>> iov_iter_is_*(), it can't eliminate and skip unreachable branches in >>>>>> following iterate*(). >>>>> >>>>> I think we need to kill the iov_iter_is_* helpers, renumber to not do >>>>> the pointless bitmask and just check for equality (might turn into a >>>>> bunch of nice switch statements actually). >>>> >>>> There are uses like below though, and that would also add some overhead >>>> on iov_iter_type(), so it's not apparent to me which version would be >>>> cleaner/faster in the end. But yeah, we can experiment after landing >>>> this patch. >>>> >>>> if (type & (ITER_BVEC|ITER_KVEC)) >>> >>> There are exactly 3 such places, and all of them would've been just as well >>> with case ITER_BVEC: case ITER_KVEC: ... in a switch. >>> >>> Hmm... I wonder which would work better: >>> >>> enum iter_type { >>> ITER_IOVEC = 0, >>> ITER_KVEC = 2, >>> ITER_BVEC = 4, >>> ITER_PIPE = 6, >>> ITER_DISCARD = 8, >>> }; >>> iov_iter_type(iter) (((iter)->type) & ~1) >>> iov_iter_rw(iter) (((iter)->type) & 1) >>> >>> or >>> >>> enum iter_type { >>> ITER_IOVEC, >>> ITER_KVEC, >>> ITER_BVEC, >>> ITER_PIPE, >>> ITER_DISCARD, >>> }; >>> iov_iter_type(iter) (((iter)->type) & (~0U>>1)) >>> // callers of iov_iter_rw() are almost all comparing with explicit READ or WRITE >>> iov_iter_rw(iter) (((iter)->type) & ~(~0U>>1) ? WRITE : READ) >>> with places like iov_iter_kvec() doing >>> i->type = ITER_KVEC | ((direction == WRITE) ? BIT(31) : 0); >>> >>> Preferences? >> >> For the bitmask version (with this patch) we have most of >> iov_iter_type() completely optimised out. E.g. identical >> >> iov_iter_type(i) & ITER_IOVEC <=> iter->type & ITER_IOVEC >> >> It's also nice to have iov_iter_rw() to be just >> (type & 1), operations with which can be optimised in a handful of ways. >> >> Unless the compiler would be able to heavily optimise switches, >> e.g. to out-of-memory/calculation-based jump tables, that I doubt, >> I'd personally leave it be. Though, not like it should matter much. > > The advantage of the bit-masks is that the 'usual' options can > be tested for together. So the code can be (for example): Well, you can do that for the non-bitwise case as well. In a simpler form but should be enough. enum { ITER_IOVEC = 1, ITER_BVEC = 2, ... } if (type <= ITER_BVEC) { if (iovec) ... if (bvec) ... } else { ... } > if (likely(iter->type & (ITER_IOVEC | ITER_PIPE) { > if (likely((iter->type & ITER_IOVEC)) { > ... code for iovec > } else [ > ... code for pipe > } > } else if (iter->type & ITER_BVEC) { > ... code for bvec > } else if (iter->type & ITER_KVEC) { > .. code for kvec > } else { > .. must be discard > } -- Pavel Begunkov